The weight of materials contained in large storage vessels is often measured by attaching strain sensors or "extensometers" to the support structure of such vessels. By measuring the amount of compression in such support structures as material is placed in the vessel, the weight of such material can be determined.
In general, the strain sensor or extensometers used in such weighing systems measure strain in only one dimension, normally along the longitudinal axis of the support structure. An example of a strain sensor used for this purpose is described in U.S. Pat. No. 4,064,744. The principle disadvantage of a strain sensor of this type results from temperature-related effects on the sensors. When the temperature of the support structure increases, the support structure expands. It is, of course, undesirable for a strain sensor mounted on the support structure to respond to such expansion of the support structure since the expansion is unrelated to weight-induced strain. Therefore, extensometers or strain sensors are normally fabricated from the same material as the support structure on which they are mounted so that they will expand at the same rate as the support structure.
In weighing applications, however, even a small mismatch in the coefficients of thermal expansion, such as are incurred because of unpredictable variations in the composition of the material forming the support structure and strain sensor, or to differences in how they are fabricated, will cause noticeable errors in the measurement which cannot be easily corrected. Further errors are caused by variations in the rate at which the temperature of the support structure varies relative to the strain sensor. As a result, a temperature differential between the suport structure and strain sensor develops, thereby causing the strain sensor to generate a temperature-induced error. This error will continue as long as there exists a temperature differential between the support structure and strain sensor.
Another area of potential difficulty arises from the need to measure the stress in support structures having a curved outer surface. In the event that stress-induced strain is measured by extensometers having four mounting points, it is not possible for the four points to all lie on the curved surface.
Another difficulty encountered with extensometers arises from the need to accurately position the mounting point for the exensometers. Extensometers are often bolted to support structures through threaded bores machined in the structure. Inaccurately positioning the bores applies a pre-strain to the extensometer, thereby degrading its performance.